Power-on reset circuit

Abstract: A power-on reset circuit for providing the stable generation of a reset signal without being affected by the rising characteristic of a power-supply voltage. In the novel power-on reset circuit, a flip-flop (13) composed of a pair of cross-connected inverters (11, 12) is used, a common connection point between a diode (Q15) and a capacitor (C11) connected to each other in series is connected to a node (N11), which is one output of the flip-flop, an output signal at a node (N12), which is another output of the flip-flip, is supplied to a reset pulse generator (16) via an inverter (15), a change in the output signal is detected by the reset pulse generator, and a reset pulse is generated based on the detected signal change.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power-on reset circuit that generates a reset signal when a power is turned on and, more particularly, to a power-on reset circuit that generates a reset signal when a supply voltage has reached a certain threshold level. 2. Description of the Related Art A power-on reset circuit having a constitution shown in Fig. 1 has been proposed as a type of the power-on reset circuit mentioned above. Referring to Fig. 1, a resistor R1 and a capacitor C1 are connected to each other in series between Vcc power and ground. A node N1, a common connection point between the resistor R1 and the capacitor C1, is connected to an input terminal of an inverter 1. The inverter 1 is composed of a p-channel MOS (Metal Oxide Semiconductor) transistor Q1 and an n-channel MOS transistor Q2, both of which are connected to each other in series between power-supply voltage Vcc and ground, gates of the transistors being commonly connected to each other. In the inverter 1, a gate common connection point between the MOS transistors Q1 and Q2 provides an input terminal, which is connected to the node N1, while a node N2, which is a drain common connection point between the transistors, provides an output terminal from which a reset signal is outputted. An operation of a power-on reset circuit of related art having the above-mentioned constitution will be described. First, when the Vcc power is turned on, the power-supply voltage Vcc goes up. When a potential difference relative to the node N1 has exceeded a threshold voltage of the p-channel MOS transistor Q1, the MOS transistor Q1 is turned on. In the initial stage of this power-on operation, a charging voltage of the capacitor C1 remains low. When the p-channel MOS transistor Q1 is turned on, a potential of the node N2 goes high, outputting a reset signal. Then, as time passes, the charging voltage of the capacitor C1 increases and when the potential of the node N1 exceeds a threshold voltage of the n-channel MOS transistor Q2, the MOS channel transistor Q2 is turned on. This causes the potential of the node N2 to go low, thereby preventing the reset signal from being outputted. However, in a power-on reset circuit of related art having the above-mentioned constitution, when the power voltage Vcc is quickly raised, the reset signal is stably generated by the above-mentioned circuit operation; when the power voltage Vcc is very slowly raised, the potential of the node N1 exceeds the threshold voltage of the n-channel MOS transistor Q2 before the potential difference between the power-supply voltage Vcc and the node N1 exceeds the threshold voltage of the p-channel MOS transistor Q1, thereby failing to generate the reset signal. EP-A-0 150 480 and EP-A-0 421 124 describe power-up reset pulse generating circuits comprising cross-connected inverters. In these circuits, each inverter consists of a pair of transistors of opposite polarity connected in series between the power supply terminal and ground. US-A- 4 558 233, figure 3 discloses a power- on reset circuit with an inverter having a load, a transistor and a diode in series between a power supply node and a reference potential, the diode increasing the inverter threshold voltage. US- A- 4 013 902 discloses a power- on reset circuit with two cross- coupled inverters made up of a load resistor, a transistor and a diode in series, one diode being connected to a power supply node and the other diode being connected to ground. OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a power-on reset circuit that generates a reset signal with stability without being affected by a rising characteristic of a power-supply voltage. In carrying out the invention, there is provided a power-on reset circuit comprising: a first power supply node; a reference potential node; a flip-flop composed of first and second cross-connected inverters, an output of the first inverter providing an input to the second inverter and an output of said second inverter providing an input to said first inverter; a series-connected circuit composed of a diode and a capacitor connected to each other in series between said first power supply node and said reference power supply node, a common connection point between said diode and said capacitor being connected to the output of one of the first and second inverters; and a reset signal generator for generating a reset signal based on a change in an output signal of one of the first and second inverters; characterised in that: each one of the first and second inverters comprises a load, a metal insulator semiconductor transistor and a diode connected in series between the first power supply node and the reference potential node, the load of the first inverter being connected to the first power supply node, the load of the second inverter being connected to the reference potential node, the diode of the first inverter being connected to the reference potential node and the diode of the second inverter being connected to the first power supply node. In carrying out the invention and according to another aspect thereof, there is provided the power-on reset circuit wherein the above-mentioned flip-flop is composed of a pair of inverters whose load is made up of a high resistance element. Accordingly, in the power-on reset circuit, the flip-flop has only one stable point when a supply voltage is sufficiently low and two stable points when the supply voltage is sufficiently high. This flip-flop recognises the state in which the supply voltage is high and the state in which the supply voltage is low. The reset signal is generated between the two states. In this set-up, the reset signal can be generated even if the supply voltage rises very slowly, thereby providing a stable reset-signal generation independently of how the power supply rises. Since the resistors used in the inverters constituting the flip-flop of this power-on reset circuit are made of high resistance elements, a leak current can be suppressed to a sufficiently low level. Since the diode is connected in series to the driving transistors of the inverters constituting the flip-flop, the driving transistors of the output-side inverter will not turn on when the supply voltage drops quickly to 0 V, thereby making it possible to generate the reset signal when the power voltage has risen again. The above and other objects, features and advantages of the present invention will become more apparent from the accompanying drawings, in which like reference numerals are used to identify the same or similar parts in several views. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram illustrating a power-on reset circuit of the related art; Fig. 2 is a diagram illustrating a power-on reset circuit practised as one preferred embodiment of the invention; Fig. 3 is a diagram illustrating waveforms of nodes obtained when a rising time of power-supply voltage Vcc of the power-on reset circuit of Fig. 2 is 5 ms; Figs. 4(A) to 4(C) are diagrams illustrating potential variations of node N11, node N12 and node N13 relative to the power-supply voltage Vcc respectively; and Fig. 5 is a diagram illustrating waveforms of the nodes obtained when the power-supply voltage Vcc has fallen temporarily and then risen again. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention will be described in further detail by way of example with reference to the accompanying drawings. Referring to Fig. 2, there is shown a circuit diagram of a power-on reset circuit practised as one preferred embodiment of the invention. In this embodiment, a MOS (Metal Oxide Semiconductor) transistor is used for a MIS (Metal Insulator Semiconductor) transistor for example. Referring to Fig. 2 again, a resistor R11 as a load, a driving n-channel MOS transistor Q11, and a diode-connected n-channel MOS transistor Q12 are connected in series between Vcc power supply (a first power supply) and ground (a second power supply), thereby constituting an inverter 11. Likewise, a diode-connected p-channel MOS transistor Q13, a driving P-channel MOS transistor Q14, and a resistor R12 as a load are connected in series between Vcc power supply and ground, thereby constituting an inverter 12. These inverters 11 and 12 are cross-connected to each other to constitute a flip-flop 13. That is, a node N11, which is an output of the inverter 11, is connected to a gate of the driving p-channel MOS transistor Q14 of the inverter 12; a node N12, which is an output of the inverter 12, is connected to a gate of the driving n-channel MOS transistor Q11 of the inverter 11. In this flip-flop 13, values of the resistors R11 and R12 serving as loads for the inverters 11 and 12 are on the order of several giga ohms (GΩ)in order to suppress a leak current to a sufficiently low level (down to nano ampere (nA) level). Between Vcc power supply and ground, there is connected a series-connected circuit 14 composed of a capacitor C11 and a diode-connected n-channel MOS transistor Q15. A common connection point of the series-connected circuit 14 is connected to one of outputs of the flip-flop 13, for example the node N11 of the inverter 11. It should be noted that a boundary between two stable states of the flip-flop 13 is determined by the values of the resistors R11 and R12 and subthreshold currents of MOS transistors Q11 through Q15. The other output of the flip-flop 13, that is, a point between the node N12 and ground of the inverter 12, is connected to a capacitor C12. One of output signals of the flip-flop 13, for example an output signal coming from the node N12 of the inverter 12, is supplied to a reset pulse generator 16 via an inverter 15 based on a change in the signal. The reset pulse generator 16 comprises an inverter 17 for inverting a signal at a node N13, which is an output of the inverter 15, a delay circuit 18 for delaying the inverted signal coming from the inverter 17 for a predetermined period of time, and a NOR gate 19 in which the signal at the node N13 and the signal delayed by the delay circuit 18 are entered. Based on a difference between these signals, a reset pulse is generated. A width of the reset pulse is determined by the delay time of the delay circuit 18. For the delay circuit 18, a circuit having a constitution in which inverters are concatenated in an amount corresponding to the delay time. Now, an operation of the above-mentioned embodiment will be described. For simplicity, it is assumed that the rising time of the power-supply voltage Vcc is infinite. First, when Vcc = 0 V, the potentials at the nodes N11 and N12 are both 0 V. If the power-supply voltage Vcc is sufficiently low, impedance of the MOS transistors Q11 through Q15 is sufficiently higher than that of the resistors R11 and R12, so that the flip-flop 13 has only one stable point. That is, the potential at the node N11 is fixed to Vcc level and the potential at the node N12 to ground level. When the power-supply voltage Vcc gets near a threshold value of the inverter 12, the impedance of the MOS transistor Q13 and Q14 becomes generally equal to that of the resistor R12, causing the potential at the node N12 to shift from low level to high level. When the potential at the node N12 has reached a potential sufficient for the MOS transistor Q11 to turn on, the MOS transistor Q11 turns on, the potential at the node N11 goes low, and the content of the flip-flop 13 is inverted. The inversion (shifting of the potential at the node N12 from low to high level) of the content of the flip-flop 13 is transmitted to the reset pulse generator 16 via the inverter 15. This causes the reset pulse generator 16 to generate a reset pulse. That is, when the potential at the node N12 shifts from low to high level, a signal that shifts from high to low level appears at the node N13. In the reset pulse generator 16, the signal at the node N13 is inverted by the inverter 17 and delayed by the delay circuit 18 by the predetermined period of time. The delayed signal is OR-ed with the signal at the node N13 to generate the reset pulse. When the power-supply voltage Vcc gets sufficiently high, the potential at the node N11 becomes a gate voltage at which a subthreshold current of the MOS transistor Q 12 becomes equal to a current flowing in the resistor R11 and the potential at the node N12 becomes a gate voltage at which a subthreshold current of the MOS transistor Q13 becomes equal to a current flowing in the resistor R12. It should be noted that a channel length of the MOS transistor Q15 is set long and a channel length of the MOS transistor Q 12 is set standard so that the subthreshold current of the MOS transistor Q12 becomes greater than that of the MOS transistor Q15. The MOS transistor Q15 is provided only as an external trigger for prompting the inversion of the flip-flop 13, thereby making an inverted state hard to revert again. When the rising time of the power-supply voltage Vcc is finite, the potential at the node N11 is fixed to power-supply Vcc level and the potential at the node N12 to ground level by coupling the capacitors C11 and C12. Referring to Fig. 3, there are shown waveforms of the nodes N11, N12, and N13 when the rising time (Vcc = 0 V to 5 V) of the power-supply voltage Vcc is 5 ms. As clearly seen from Fig. 3, the above-mentioned constitution allows the flip-flop 13 to recognize whether the power-supply voltage Vcc is in high state or low state and generate a reset pulse at a boundary in between, thereby securely providing reset pulse generation even if the power-supply voltage Vcc takes very long to rise. Meanwhile, a variation in the potentials at the nodes N11 through N13 has a hysteresis and presents different hysteresis characteristics for rising and falling of the power-supply voltage Vcc. Referring to Fig. 4(A) to 4(C), there are shown variations of the potentials at the nodes N11 (A), N12 (B), and N13 (C) relative to the power-supply voltage Vcc. Referring to Fig. 4(C), the reset pulse is generated when the potential at the node N13 shifts from high to low as described above. Now, a circuit operation will be described that takes place when the power-supply voltage Vcc temporarily drops in a time shorter than a time of discharge by the resistors R11 and R12 such as the case of momentary turn-off of power and rises again. First, in the case where the power-supply voltage Vcc quickly drops to 0 V, the potential at the node N11 becomes about -0.7 V (a drop by pn junction) due to the supply of a charge by the pn junction of drains of MOS transistors Q11 and Q14. Then, when the power-supply voltage Vcc rises again, the transistor Q14 will turn on if the potential at the node N11 is as low as about -0.7 V. Consequently, even in a low power-supply voltage state, the potential at the node N12 becomes Vcc level and the potential at the node N13 becomes ground level, thereby preventing the reset pulse from being generated. However, in the above-mentioned preferred embodiment of the invention, the diode-connected MOS transistor Q12 is connected to the MOS transistor Q11 in series and the diode-connected MOS transistor Q13 is connected to the MOS transistor Q14 in series, so that even when the power-supply voltage Vcc quickly drops to 0 V, the transistor Q14 will not turn on. Therefore, when the power-supply voltage Vcc rises again, the reset pulse can be generated securely. It will be apparent to those skilled in the art that, although the above-mentioned embodiment uses the diode-connected MOS transistors Q12 and Q13, the embodiment can use diodes instead. Referring to Fig. 5, there are shown waveforms of the nodes N11, N12, and N13 obtained when the power-supply voltage Vcc has fallen temporarily and then risen again. It should be noted that, in the above-mentioned constitution, if the threshold voltage of the p-channel MOS transistor Q13 is Vth, the potential at the node N12 rises only up to Vcc - Vth, thereby making it possible for a rush current to flow in the inverter 15. The rush current to flow in the inverter 15 can be suppressed to a level lower than a current that flows in the resistor R12 by setting the threshold voltage of the p-channel MOS transistor constituting the inverter 15 to a level higher than the threshold voltage of the p-channel MOS transistor Q13 (for example, by setting a channel length of the transistor constituting the inverter 15 relatively long). As described and according to the invention, there is provided a power-on reset circuit using a flip-flop composed of a pair of cross-connected inverters, wherein a common connection point between a diode and a capacitor connected to each other in series is connected to one output of the flip-flop to make the flip-flop recognize states in which a power supply voltage is high and low, thereby generating a reset signal at a boundary between the states. This novel setup surely generates the reset signal even when the power-supply voltage rises very slowly, thereby providing the stable generation of the reset signal without being affected by power-supply rising time. Use of a high resistance element for a resistor for the inverters constituting the flip-flop suppresses a leak current to a sufficiently low level. Further, series connection of a diode to each of the driving transistors for the inverters constituting the flip-flop prevents the driving transistor of the output-side inverter from turning on, thereby ensuring the stable generation of the reset signal even when the power-supply voltage temporarily drops and then rises again. While the preferred embodiment of the present invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the scope of the appended claims.

Claims: A power-on reset circuit comprising: a first power supply node; a reference potential node; a flip-flop (13) composed of first and second cross-connected inverters (11, 12), an output of the first inverter (11) providing an input to the second inverter (12) and an output of said second inverter (12) providing an input to said first inverter (11); a series-connected circuit (14) composed of a diode (Q15) and a capacitor (C11) connected to each other in series between said first power supply (Vcc) node and said reference power supply node, a common connection point between said diode (Q15) and said capacitor (C11) being connected to the output (N11) of one of the first and second inverters; and a reset signal generator (16) for generating a reset signal based on a change in an output signal of one of the first and second inverters; characterised in that: each one of the first and second inverters (11, 12) comprises a load (R11/R12), a metal insulator semiconductor transistor (Q11/Q14) and a diode (Q12/Q13) connected in series between the first power supply node and the reference potential node, the load (R11) of the first inverter (11) being connected to the first power supply node, the load (R12) of the second inverter (12) being connected to the reference potential node, the diode (Q12) of the first inverter (11) being connected to the reference potential node and the diode (Q13) of the second inverter (12) being connected to the first power supply node. A power-on reset circuit as defined in claim 1, wherein when the potential of the first power supply node is different from that of the reference potential node and is low, the impedance of the transistor (Q11, Q14) and the diode (Q12, Q13) of each of said first and second inverters is sufficiently higher than that of the load (R11, R12) of each of said first and second inverters that the flip-flop has only one stable point, and when the potential of the first power supply node is sufficiently high to cause the transistor in the second inverter to turn on, an output of the flip-flop is inverted. A power-on reset circuit as defined in claim 1 or 2, wherein the loads of the first and second inverters are resistors (R11, R12). A power-on reset circuit as defined in claim 1 or 2, wherein the loads (R11, R12) of the first and second inverters are high resistance elements. A power-on reset circuit as defined in claim 3 or 4, wherein the loads (R11, R12) of the first and second inverters are sized sufficiently large to reduce leak current. A power-on reset circuit as defined in any one of claims 1 to 5, wherein said first and second inverters (11, 12) comprise an n-type metal oxide semiconductor transistor (Q11) and a p-type metal oxide semiconductor transistor (Q14), respectively. A power-on reset circuit as defined in any one of claims 1 to 6, wherein the diode (Q12, Q13) of each of said first and second inverters, connected to the corresponding transistor (Q11, Q14), is so that when the voltage (Vcc) of the first power supply node quickly drops to 0 V, the corresponding transistor (Q11, Q14) will not turn on, thus when the first power supply voltage (Vcc) rises again, the reset pulse is generated by said reset pulse generator (16). A power-on reset circuit as defined in any one of claims 1 to 7, wherein a boundary between two stable states of the flip-flop (13) is determined by the values of the loads (R11, R12) of the first and second inverters, and the subthreshold currents of the transistors (Q11, Q14) and the diodes (Q12, Q13) of the first and second inverters and the diode (Q15) of the series-connected circuit (14), said boundary corresponding to the point at which it is considered that the voltage (Vcc) of the first power supply node has switched between low state and high state. A power-on reset circuit as defined in any of claims 1 to 8, wherein the diodes (Q12, Q13) of the first and second inverters are diode-connected transistors. A power-on reset circuit as defined in any of claims 1 to 9, wherein said reset pulse generator (16) is connected to the output of said second inverter (12) through a third inverter (15), and a capacitor (C12) is connected between said reference potential node and an input of said third inverter (15). A power-on reset circuit as defined in any of claims 1 to 9, wherein said reset pulse generator (16) comprises a fourth inverter (17) for inverting an output of a third inverter (15) connected to said flip-flop (13), a delay circuit (18) for delaying an inverted signal from the fourth inverter (17) for a predetermined period of time, and a NOR gate (19) for generating a reset pulse based on the output of said third inverter (17) and the output of the delay circuit (18). A power-on reset circuit as defined in claim 11, wherein the width of the reset pulse generated by said reset pulse generator (16) is determined by the delay time of the delay circuit (18). A power-on reset circuit as defined in any of claims 1 to 12, wherein a first node (N11) is provided at a common connection between the load (R11) and the transistor (Q11) of the first inverter (11), and a second node (N12) is provided at a common connection between the load (R12) and the transistor (Q14) of the second inverter (12), so that when the potential (Vcc) of the first power supply node is 0 V, the potential at each of the first node (N11) and the second node (N12) is 0 V, and when the potential (Vcc) of the first power supply node is near a threshold value of the second inverter (12), the impedance of the transistor (Q14) and the diode (Q13) of the second inverter (12) becomes about equal to that of the load (R12) of the second inverter (12) causing potential at said second node (N12) to shift from a low level to a high level.

Inventors: NAKASHIMA KATSUYA, NAKASHIMA, KATSUYA, NAKASHIMA, KATSUYA, C/O SONY NAGASAKI CORP
Applicants: SONY CORP, SONY CORPORATION
Application year: 1994
Main classification: H03K3
IPC codes: H03K17, H03K3